A potent inhibitor of human leukocyte elastase (EC 3.4.21.37) and cathepsin G (EC 3.4.21.20) and of human trypsin (EC 3.4.21.4) has been purified from human parotid secretions. The complete amino acid sequence of this protein has been determined. The sequence suggests that the protein has two domains of about 54 amino acids, each of which contains four disulfide bonds. On the basis of a limited homology to other protease inhibitors, the antielastase and antitrypsin activities are thought to be properties of the C-terminal and N-terminal domains, respectively. The affinity of the inhibitor for leukocyte elastase is very high, suggesting a functional role for the protein in preventing elastase-mediated damage to oral and possibly other mucosal tissues.